Luminescent metaborates are described in U.S. Pat. No. 4,319,161 which is incorporated herein by reference. Of particular interest is the gadolinium line emitter, Gd.sub.y Y.sub.1-x-y Ce.sub.x MgB.sub.5 O.sub.10, wherein 0.01.ltoreq.x.ltoreq.1-y and 0.02.ltoreq.y.ltoreq.0.08 which emits in 310-315 nm range. This phosphor may be used in combination with a low pressure mercury discharge for irradiation purposes such as the photo-therapy of skin diseases like psoriasis. The conventional preparation of luminescent metaborates involves dry mixing the reactants and then subjecting the mixture to multiple firings. However, the fired cakes produced accordingly tend to be inhomogeneous and stick to the firing boats resulting in lower brightness material.